1. Field of the Invention
The present invention relates to an electric arc plasma torch.
2. Description of the Prior Art
Plasma torches, also called plasma burners, are devices which are well known per se and which allow for the production of a jet of gas in the form of plasma.
According to a conventional definition, a plasma is an ionized gas which comprises at least 10.sup.15 charged particles per cubic meter, and on average, very approximately as many electrons as positive ions.
The production of a plasma requires that a large amount of energy is applied to the gas. Various means are available to this end, of which the electric arc is the most frequently used.
In electric arc plasma torches, the arc is struck between two electrodes, between which a gas flows. The gas particles are ionized by the energy produced by the arc and the gas is converted into a plasma.
Most arc plasma torches are supplied with direct current, or more precisely, by rectified alternating current.
Electric arc plasma torches may be further subdivided into two categories, according to the type of cathode used, i.e. a hot cathode or a cold cathode.
A hot cathode is a cathode which is heated to a sufficiently high temperature so that it can, by thermionic effect, emit a number of electrons which in practice ensure the flow of the arc. On account of the high temperature necessary to produce an electron emission corresponding to an arc flow intensity sufficient to reach the required power and temperature, i,e, approximately 3000.degree. C., the number of materials which can be used to manufacture a cathode of this type is very limited. Currently, only tungsten or certain alloys of tungsten are used in practice. Consequently, arc plasma torches with hot cathodes can only operate with gases which are chemically inert with regard to tungsten, such as hydrogen, nitrogen and rare gases (argon, xenon, etc . . . ). In addition to the high price of these gases, this limitation represents a serious inconvenience for this type of torch, since it is desired to use other gases. On the other hand, these cathodes have a very low rate of wear, and consequently a very long life of several hundred hours.
The second type of arc plasma torch, i.e. torches with cold cathodes, use a copper cathode, forcibly cooled to prevent it from reaching the temperature of thermionic emission. In this type of torch, aerodynamic or magnetic means, or the two simultaneously, are often used to quickly move the foot of the arc on the cathode in order to limit the wear of the latter. Torches with cold cathodes allow for the use of practically all gases. However, the lifetimes of these cathodes remains limited to a few hundred hours in the best of the cases currently known. These lifetimes are clearly lower than those of the hot cathodes on the one hand and those of the anodes on the other hand, which currently reach several thousand hours.
U.S. Pat. No. 4,002,466 discloses a plasma torch for the reduction of metal oxides, in particular for the direct reduction of iron ores. That plasma torch comprises a tungsten cathode and an anode respectively connected in the conventional way to the negative and positive poles of an electric current source. Between the cathode and the anode there is an electrically insulated nozzle intended particularly to stabilize the arc and to prevent the return of gaseous carbon from the anode towards the cathode.